1. Field of the Invention
The present invention relates to a ridge waveguide semiconductor laser, and, more particularly, to a ridge waveguide semiconductor laser mainly used for optical communications.
2. Background Art
In recent years, optical fiber communications have rapidly spread, so that semiconductor lasers for communications have found wide applications in this field from trunk systems to subscriber systems. Furthermore, with the rapid spread of the Internet and the advancement of network computing, there has been an increasing need to provide a high-speed low-cost LAN (Local Area Network) by use of optical fiber. As such, there has been also an increasing need to enhance the speed and the temperature characteristics of semiconductor lasers for data communications, which are a key device of such LANs. Several semiconductor lasers have been proposed to satisfy the above need, as described in Japanese Laid-Open Patent Publications Nos. Hei07-240564 and 2000-261093 and the specifications of U.S. Pat. No. 5,381,434 and U.S. Pat. application (laid-open) No. 2002/0113279.
Reducing the cost of an optical communications apparatus or optical communications system for data communications requires a highly reliable semiconductor laser having a long life capable of operating at high speed and high temperature without the need for a cooling system.
FIG. 8 is a perspective view of a conventional ridge waveguide type semiconductor laser, and FIG. 9 is a plan view of the ridge waveguide type semiconductor laser shown in FIG. 8.
Referring to these figures, an n side electrode 51 is provided on the rear surface of an n-InP substrate 50. On the other hand, an epitaxial layer is formed on the top surface of the n-InP substrate 50. The epitaxial layer is made up of: an n-InP cladding layer 52; an MQW (Multiple Quantum Well) active layer 53 including AlGaInAs strained quantum wells; a p-InP cladding layer 54; and p-InGaAs contact layer 55. Two grooves 56 and 57 are provided in the epitaxial layer such that they run in parallel with each other like stripes. A ridge-shaped waveguide 58 is formed on the area sandwiched by the grooves 56 and 57. Furthermore, an SiO2 (silicon dioxide) film 60 having a thickness of approximately 400 nm is formed on the entire top surface except for an opening 59 provided above the waveguide 58; that is, it is formed on the p-InGaAs contact layer 55 and the grooves 56 and 57.
A p side electrode 61 is provided over the opening 59 such that it is in contact with the p-InGaAs contact layer 55 thereunder through the opening 59. It should be noted that the p side electrode 61 is made up of a Ti (titanium) vapor deposition electrode(not shown) having a thickness of approximately 50 nm and an Au (gold) vapor deposition electrode (not shown) having a thickness of approximately 200 nm.
An Au-plated electrode 62 having a thickness of approximately 3 μm is formed on the p side electrode 61. Furthermore, an electrode lead-out line 63 is formed such that it extends from the Au-plated electrode 62. The end of the electrode lead-out line 63 constitutes a bonding pad portion 64. Such a structure enhances the heat dissipation characteristics of the electrode portion as well as facilitating pressure-bonding of leads of Au, etc. to the bonding pad portion 64.
If a positive (+) bias and a negative (−) bias are applied to the p side electrode 61 and the n side electrode 51, respectively, a current flows through the waveguide 58 predominantly. As a result, electrons and holes are injected into an active region 67 right under the waveguide 58, leading to light emission due to electron-hole recombination. Then, if the current is increased to more than a threshold value, induced emission begins, which will lead to laser oscillation, emitting laser light L′.
Generally, the end faces of a semiconductor laser resonator are obtained through crystal cleavage. That is, the element constituting each semiconductor laser is cut from a semiconductor substrate by way of crystal cleavage. In order to obtain good cleavage characteristics, conventional semiconductor lasers have a structure in which the Au-plated electrode 62 is not disposed near the cleaved surfaces, and furthermore the distance R′ from the resonator end faces 65 and 66 to the respective near edges of the Au-plated electrode 62 is set to as long as approximately 50 μm, as will be described in detail below.
The angle of each cleaved surface with respect to the wafer-reference plane usually varies from its target value; that is, they may be displaced with respect to the electrode pattern. Therefore, if the cleaved surfaces are displaced toward the Au-plated electrode 62 so that the distance R′ from the Au-plated electrode 62 to the resonator end faces 65 and 66 becomes too short, the cleaved surfaces may cross the Au-plated electrode 62, making it difficult to obtain good cleaved surfaces since the stress produced at the time of cleavage is absorbed by the Au-plated electrode 62.
For example, if the distance R′ from the resonator end faces 65 and 66 to the respective near edges of the Au-plated electrode 62 is set to 10 μm or less, the cleaved surfaces may tend to cross the Au-plated electrode.62 due to their displacement with respect to the electrode pattern. The Au-plated electrode 62 generally has a thickness of approximately between 2 μm and 4 μm. Therefore, when a cleaved surface has crossed the Au-plated electrode 62, the stress is absorbed by the Au-plated electrode 62, making it impossible to obtain a good cleaved surface.
On the other hand, to achieve a high modulation rate at high temperature, such semiconductor lasers are configured such that the length of the resonator is short and the reflectance levels of the resonator end faces are high, as compared with conventional semiconductor lasers. For example, to achieve a high modulation rate of approximately 10 Gbps at 85° C., which is required for semiconductor lasers for data communications in recent years, the resonator length has been reduced from approximately 300 μm (conventional value) to approximately 200 μm. As for the reflectance levels of the resonator end faces, the reflectance level of the front end face has been increased from approximately 30% (conventional value) to approximately between 60% and 65%, while that of the rear end face has been increased from approximately 60% (conventional value) to approximately 95%.
Increasing the reflectance levels of the resonator end faces, however, makes it difficult for the emitted light to leave the resonator, increasing the value of the current (operational current) necessary to obtain a predetermined optical output. Especially, since conventional ridge waveguide type semiconductor lasers employ the Au-plated electrode 62 having a thin film thickness (not more than 200 nm), the current density in the electrode is high, imposing a large heat load on the element due to heat generation.
Further, if the edges of the Au-plated electrode 62 are set far away from the resonator end faces 65 and 66, as described above, the distance which current must flow in the thin p side electrode 61 to reach the neighborhoods of the resonator end faces 65 and 66 increases, thereby increasing the current density of the portions of the vapor deposition electrode near the edges of the Au-plated electrode 62. As a result, the temperature increases locally at these portions, reducing the reliability and the life of the device.